1. Field of the Invention
The present invention relates to a method and a device for space division multiplexing/demultiplexing of radio signals which are organized in frames and are transmitted in duplex on the same frequency and time channel, requiring no setting. It applies, in particular, to cellular radio communications between at least one base station and a plurality of mobile units.
2. Discussion of the Background
With the constant increase in demand for mobile communications and the limitation on the number of channels allocated to the operators of cellular radio communications networks, saturation problems will become critical in future years.
Conventional techniques currently make it possible to multiplex communications in frequency division (FDMA), in time division (TDMA) or in code division (CDMA).
FDMA (Frequency Division Multiple Access) consists in allocating a pair of frequencies to each communication (one for the down path and one for the up path). On its own, this very simple multiplexing technique affords substantially no opportunity for improving spectral efficiency.
TDMA (Time Division Multiple Access) consists in allocating a precise time interval to each communication. On its own, this technique which has already been optimized to improve the spectral efficiency within the scope of wireless transmission by utilizing the gaps present in the speech signal, gives little likelihood of further improvement in this regard.
CDMA (Code Division Multiple Access) consists in allocating each communication a code defining a frequency hop law over short time intervals. Although it does allow some improvement in the spectral efficiency (progressive saturation by signal degradation rather than abrupt saturation by service interruption, as in the case of the previous two techniques), this method remains involved and expensive to implement.
A number of ways of improving the capacities of cellular communications systems are currently being investigated, namely: diversification of the cells and the waveforms, with the use of smaller and smaller cells ("microcell" and "nanocell" of the ATDMA calibration project, referring to "Advanced Time Division Multiple Access", for urban areas), and in particular a new multiplexing method which can be combined with existing techniques: Space Division Multiple Access (SDMA).
This technique is based on the idea that the radio signals corresponding to a plurality of links between a base station and mobile units may, in many cases, be spatially decorrelated.
Utilizing this decorrelation makes it possible to separate the spatial channels (even if they all occupy the same frequency and time channel) using a multipath transmission/reception system associated with an antenna base tailored to the frequency range which is used.
A number of methods using this concept have already been proposed. A first method is based on techniques for estimating direction of arrival, such as the system described in the patent by Richard Roy (US patent PCT/US92/10074, December 1991, entitled "Spatial Division Multiple Access Wireless Communication System"). A second method uses cyclostationarity characteristics of digital transmission signals to separate them by blind processing operations (U.S. Pat. No. 5,260,968, of Sep. 11, 1993, entitled "Method and Apparatus for Multiplexing Communications Signals through Blind Adaptive Spatial Filtering" by William A. Gardner and Stephan V. Schell).
For the first method, estimation of the directions of arrival of various radio sources on the basis of the signals received on an antenna base requires perfect control over the wavefront, which leads to the constraint of calibrating the antenna base which is used. Further to the extra cost introduced by setting, the standard deviation of the indirect estimate of the direction vector on the basis of the estimated direction of arrival and the setting table of the antenna base is much greater than that obtained by direct estimation methods. The direction vector corresponds to the value taken by the transfer function of the antenna base for the azimuth, elevation and carrier frequency of the incident electromagnetic wave transmitted by a source. This information is then used by the space division multiplex processing.
For the second method, 2nd order separation techniques using cyclostationarity require either differences in symbol rates or shifts in carrier frequencies of the signals to be separated in order to operate. These limitations make these techniques ill-suited to the channel configurations formed in radio communications between a base station and mobile units having decorrelated multiple routes which cannot be separated by the above methods and are therefore inoperable in most urban areas.
Lastly, to separate the various signals received, the antenna processing techniques mentioned above employ a so-called purely spatial filtering structure constituted by one complex gain per reception path. The anti-scrambling capacities of this type of structure, in terms of the number of independent interference sources which can be rejected (two decorrelated routes originating from the same source constitute two independent interference sources) are suboptimal. This is because the spatial filter adapts to one useful route followed by a source and rejects all the other routes, both the decorrelated useful routes followed by this source and the routes followed by the interference signals.
In the rest of the description, the terms "sensors" and "antennas" will be used interchangeably, as will the terms "multisensor", "multipath" and "multisensor array" and "antenna base", it being moreover known that an antenna base, or multisensor array, forms a number of transmission paths equal to the number of antennas or sensors.